1. Field of the Invention
The present invention relates to the plastics industry in general and in particular to apparatus for extruding blown film. More particularly, it relates to post extrusion devices generally known as bubble cages or guides.
2. Related Art
All blown film is extruded either vertically, up or down, or horizontally. In all instances, once the polymer exits the cylindrical die, it forms a cylindrical "bubble" and is drawn from the die by means of two rollers (usually known as "nip rollers") which contact a collapsed outer end of the bubble. The distance between the die surface and the nip rollers may vary from six to over one hundred feet. As it exits the die, the bubble is inflated with air to the desired diameter. Normally, the air inflates the bubble through the die and once the requisite diameter has been reached inflation ceases and the air is trapped between the face of the die and the nip roller.
The bubble must be guided to ensure good centering and tracking with minimum side-to-side oscillation or off-centre movement. The guiding has been accomplished by means of bubble cages; individual guiding bars; guiding bars interconnected by means of chains, gears and the like; and a variety of other methods.